The semiconductor manufacturing industry makes extensive use of projection printers which feature a high degree in optical precision with the capability of resolving state-of-the art circuit element details having line widths of less than two microns (2.times.10.sup.-6 meters).
Typical photo-imaging processes involve the use of photomasks for standard, high volume production runs and of photomasks for specialized, low volume runs. The patterns on photomasks for standard products are advantageously protected by coverplates. Coverplate protection of photomasks is found to be highly desirable in that coverplate-protected photomasks can be expected to last for the entire commercial life cycle of the respective product. In contrast, photomasks for specialized, short-run products often are not protected by coverplates. A major reason for omitting the coverplates from photomasks for newly introduced short-run products is that first generation devices may still include deviations from design requirements. Redesigns may become necessary. In such instances, a fast turn-around time is often essential for the timely commercial introduction of the new devices.
However, intermixing such unprotected photomasks with photomasks protected by state-of-the-art coverplates in consecutive photo-imaging runs of a projection printer brings about time and cost related problems. Each time the operation of the projection printer changes from the use of coverplate-protected photomasks to the unprotected photomasks or back to the operation with the coverplate protected photomasks, an intricate adjustment to the optics of the projection printer becomes necessary. A full cyclical switch from coverplate protected photomasks to unprotected photomasks and back to protected photomasks typically requires about one workday which is then lost to the use of the projection printer for production purposes. Such production losses are highly undesirable.